The present invention relates to an electrode element and, in particular, to a neural stimulator element and lead for installation in a patient's epidural space to stimulate the patient's dura.
Neural stimulator electrodes and leads have been employed in the past which are installed in the epidural space of the spinal cord of a patient for stimulating selected locations along the spinal cord for such purposes as the relief of pain. As disclosed in U.S. Pat. No. 4,285,347 (Hess), it is generally advantageous to selectively direct the electrical current from such electrode directly against the dura in the spinal cord, rather than conduct the current omnidirectionally from the electrode. Selective direction of the current substantially reduces the power requirements and, thereby, substantially extends the life of the power generating source. This is advantageous particularly where the power generating source is subcutaneously implanted. Selective direction of the current and reduction of the current also reduces the possibility of side effects which might result from stimulation of undesired or unnecessary tissue.
It is important that the electrode and lead be stabilized after installation and fixation in the epidural space to prevent shifting of the electrode. It is also important to prevent rotation of the electrode particularly where the electrode is directional. Such shifting or rotation may occur, for example, during strenuous activity or exercise by the patient. If the electrode becomes rotated away from the dura at the site of stimulation, its desired effect will be greatly diminished and may be lost altogether. Moreover, other bodily tissue which need not be stimulated may instead be stimulated resulting in possible undesirable side effects.
In the aforementioned U.S. Pat. No. 4,285,347, an epidural lead stabilizing construction is disclosed which comprises three curved loops at the distal end of the lead which extend in three dimensions to effect the sought after stabilization. Although the stabilization produced by this curved loop construction may be satisfactory in many lead installations, it does have at least one disadvantage. Due to the presence of the three curved loops, a total of three stable rotational positions are possible. One of these positions is the desired position in which the electrode and two of the loops are in generally coplanar relationship to each other and lie against the dura to stabilize the directional electrode directly against the dura. The other two positions are caused by tilting of the third loop if it bears against a vertebra. When the third loop tilts one way or the other from being perpendicular to the dura, one of the remaining loops will bear against the dura, but the other loop and the electrode will also tilt so as to be angled away from the dura. These latter two positions are not desired because the directional electrode will be rotated out of direct contact with the patient's dura.
Neural stimulator elements and leads incorporating the principles of the present invention overcome these several aforementioned disadvantages. In an electrode element or lead of the present invention, a stylet need not be relied upon to retract or otherwise directly operate the stabilizers of the present invention. The stabilizers of the present invention are quite flexible in a direction parallel to the axis of the element. Thereby, the stabilizers readily flex into an inactive reclined position as they are passed through the implantation needle, such as a Tuohy needle, and through the epidural space of the patient. However, once the element has been located at its desired location at the dura, slight movement of the element in its reverse direction will readily cause the stabilizers to assume their stabilizing position and, in this position, will prevent shifting and rotation of the lead and electrode element. In a neural stimulator electrode element and lead of the present invention, the number of possible stable rotational positions is reduced to only two, only one of which is undesirable, thereby facilitating placement and stabilization of the electrode against the dura.
In one principal aspect of the present invention, an electrode element for a neural stimulator lead includes an elongate sheath and a passage in the sheath extending axially thereof which is adapted to receive an electrical conductor. The electrode is positioned in the passage and communicates through the exterior wall of the sheath. A pair of flexible elongate members on the exterior wall of the sheath extend laterally from the sheath at a substantial angle to the axis of the elongate sheath. Each of the members is attached at one end to the exterior wall of the sheath and the other end is free. The flexible members are capable of flexing adjacent their attached end so as to move between their laterally extending position to a position adjacent the exterior wall of the sheath and substantially parallel to the axis of the sheath.
In another principal aspect of the present invention, each of the aforementioned flexible members has a width which is substantially greater than its thickness and the width extends substantially perpendicular to the axis of the sheath.
In still another principal aspect of the present invention, when the aforementioned flexible members are in their laterally extending position, they extend substantially perpendicular to the axis of the sheath.
In still another principal aspect of the present invention, the aforementioned electrode communicates through a portion only of the perimeter of the exterior wall of the sheath and the flexible members are attached to the exterior wall of the sheath at approximately 90.degree. from the portion through which the electrode communicates.
In still another principal aspect of the present invention, the perimeter of the aforementioned sheath is smaller adjacent the flexible members than it is in the remainder of the sheath.
In still another principal aspect of the present invention, the aforementioned flexible members are between the distal end of the sheath and the electrode.
In still another principal aspect of the present invention, in the aforementioned electrode elements at least one opening extends through the exterior wall of the sheath at the electrode for communicating the electrode with the exterior wall.
In still another principal aspect of the present invention, coupling means is included in the aforementioned electrode elements adjacent the end of the sheath for coupling the element to the neural stimulator conductor and the conductor is combined therewith.
In still another principal aspect of the present invention, two pairs of the aforementioned flexible members are provided, each of the pairs being spaced from each other along the axis of the sheath by a distance greater than the length of each flexible member.
In still another principal aspect of the present invention, the aforementioned electrode elements are the tip for the neural stimulator lead.
These and other objects, features and advantages of the present invention will be clearly understood through a consideration of the following detailed description.